Japan has become the first country to ban imports of British poultry amid growing concerns about the threat of avian flu. The announcement comes as more than 15,000 chickens face slaughter in Norfolk after the virus was discovered on a further two farms over the weekend.

The ban is an attempt to prevent the spread of avian flu to Japan's domestic birds. Hundreds of thousands of birds were culled last year as the H5N2 strain of avian flu was found in farms near Tokyo.

Though the movement of poultry and captive birds will be restricted in a one-kilometre exclusion zone around the affected farms in Norfolk, officials sought to reassure members of the public yesterday that the countryside was still open for business. David Collinson, Norfolk county council's head of trading standards, said: "Every single road and footpath is open and this is nothing to do with the eggs or birds bought from any shop in or outside the zones. It does mean that vehicles passing through the restricted zone carrying poultry or captive birds should not stop within the zone. The orders advise all bird-keepers within a declared zone to maintain high standards of biosecurity, and any movements of poultry and other captive birds within the zone must be licensed by a veterinary inspector."

Early tests suggest that the virus in the new cases is related to the H7N3 avian flu found at Witford Lodge farm in Hockering last week. The two farms involved are Norwich Road farm at North Tuddenham and the nearby Mowles Manor poultry unit. Neither is linked to Banham Poultry, owner of Witford Lodge, which is a few miles from the new cases. Experts are attempting to trace links between the three farms involved.

The H7 virus is not as dangerous to human health as the H5N1 avian flu strain which has so far claimed the lives of more than 100 people in Asia. "We are investigating any links and movements between these farms," the Department for Environment, Food and Rural Affairs said.

The new cases have struck in the heartland of Britain's poultry industry, which produces many of the 850 million chickens and 9bn eggs consumed by Britons every year.

One of the newly affected farms has 7,500 free-range chickens and the other 7,800. These will be slaughtered. About 35,000 chickens have already been culled at Witford Lodge. These were not destined for consumption but infected birds will have passed on their viruses to their eggs, which could be in circulation in farms owned by Banham Poultry. Because H7 has a low mortality rate, birds can carry and spread the virus without dying.

The government's chief veterinary officer, Debby Reynolds, said: "We still cannot say whether either of these two further farms are the index case; further premises may be involved."

Defra said the working hypothesis for the source of the virus was that it came from outside the farms or from wild birds.

A poultry worker at Witford Lodge developed conjunctivitis after being exposed to the H7 virus last week, but the Health Protection Agency said that no other workers had shown signs of illness.

Ian Jones, director of research at Reading University, said: "Only one human death has been ascribed to H7 influenza, in the Netherlands in 2003, so while this is another form of bird flu it is not H5N1 and unless something very unexpected has happened to the virus it does not pose the same level of threat."